Apparatus for determining the position of the conductor in electrical cables



March 3, 1942- M. F. PETERS ET AL ITION OF THE CABLES 8 APPARATUS FOR DETERMINING THE POS CONDUCTOR IN ELECTRICAL Filed Oct 18, 195

INVENTORS MELV/LLE F. PETERS BY THEODORE B. GODFREY ATTORNEY iatented Mar. 3, 1942 APPARATUS FOR DETERMINING THE POSI- TION OF THE CONDUCTOR IN ELECTRI- CAL CABLES Melville F. Peters, Beltsville, and Theodore B.

Godfrey, Bethesda, Md.

Application October 18, iasaseriai No. 235,700 screams. (01.177-311) (Granted under the act of March3, 1883, as

amended April 30, 1928; 370 0. G.

This invention relates to devices for determining the position of the conductor in electrical cables, and more particularly to an apparatus that depends upon a capacitance method for making such determinations.

It is highly desirable that the conductor of an electrical cable be positioned in the center of its surrounding insulation, especially if the cable is intended for an exacting purpose, such as the ignition circuits of'automobile or aviation engines.

If several electrodes, preferably of the same Ill shape, are disposed about a cable containing an electrical conductor, the capacitances between the electrodes and the conductor depend upon the position of the conductor, and its position may be deduced from the relative values of the capacitances, provided there is noangular variation in the overall dielectric constant of the insulating material of the cable. If the electrodes are of identical shape and are disposed symmetrically about the cable, the capacitances are all equal when the conductor is at the center of the cable.

One of the objects of this invention is to provide an apparatus that will determine whether or not the conductorin a cable is properly centered.

Another object of the invention is to provide an apparatus tha'twill indicate visually to the operator of the apparatus, whether the conductor of a cable is centered, or in what direction and approximately to what extent it is on center.

A further object of this invention is to provide an apparatus that may be'used to actuate means for marking portions .of a cable wherein the conductor is oil-center, or for controlling the position of the conductor as the cable isbeing extruded.

A still further object of the invention is to provide an apparatus that may be used to actuate a recording means for showing the position of the conductor along the length of the cable. With such objects in view, as well as other advantages which may be incident to the use of Fig; 2 is a'wiring diagram of one of the lm-. pedance bridges of the circuit of Fig. 1, shown in a more conventionalized form.

Referring to the drawings,

electrodes 4, 5, 4' and 5', spaced about the cable 3. Theelectrodes 4,5, l, 5, are approximate quadrants of a cylindrical shell of a convenient length. To test a length of the cable 3, its conductor 6 is grounded, and the cable 3 is drawn through the electrodes I, 5, 4', 5', which are held stationary.

The upper part of the electrical circuit shown in Fig. 1 consistsof two impedance bridges, the arms of the bridge on the left being the resistors l and 8, and the capacitances between electrodes usually balance out.

the apparatus, the invention consists of the parts and combinations thereof hereinafter set forth and claimed, with the understanding that. the several necessary elements constituting the same may be varied in proportion and arrangement without departing from the nature and scope'of the invention, as defined in the appended claims. In the accompanying drawing:

Fig. 1 is a schematic wiring diagramshowing the preferred arrangement or our apparatus.

4 and 5 and the conductor 6 bf the cable 3. Connected in parallel with the capacitances are the small variable capacitors 9 and II) which are inserted for balancing the bridge, since stray capacitances in the wiring of The bridge isgrounded at point ll. Irf Fig. 2 this bridge is shown in the more conventional form, like reference characters being used. The bridge on the right of Fig. 1 is made up of the same components as are present in the bridge on the left, corresponding parts being designated by the same number primed.

The two bridges have a common source of alternating power l2, which may be a vacuum tube oscillator. It is connectedto the bridges at the points l3 and L3. The frequency of the oscillator-J2 andthe resistance of resistors I, 8,1 and 8', are best fixed at such value that the condition for maximum variation in potential at points M, l5, and l4, IS, with variations in capacitances of electrodes 4, 5, 4', 5', respectively,.is fulfilled, namely, that for any one side .of a bridge, such as, for example, those components included between the points l3, l4 and II of the bridge on the left, 'RCo=1' where R is the re.-

sistance I, C is the capacitance in the arm between points H and II, and 'w is equal to 21r f,-f being the frequency of the oscillator l2. The alternating potentials of points l4,.l5, l4, l5 are rectified by rectifiers l6, l1, l6, ll, re-

' spectively. Vacuum tube rectifiers are indicated in the drawing, but rectiflers'of other types may be used. The rectified potentials originating in the bridge on the leftzof the diagram are differentially amplified by amplifying tubes [8 and IS. The rectified potentials originating in the bridge on the right are differentially amplified by amplifying tubes I8 and IS. The bridge arrangement of the amplifying tubes aids in there is shown in Fig. 1 a cross-section through a cable 3, and

the bridge will not of amplification is advisable if it is desired to detect readily a displacement of a conductor from the center of a cable by an amount equal to of the radius 01 the cable.

The rectified and amplified voltage from point l4 contr is the potential of deflection plate 2|} of a ca bode-ray oscillograph. Similarly, the potentials of deflection plates 2|, 20', 2| are determined by thepctentials'at points l5, I4,

I5 respectively. Since the potentials at points i4, I5, I4, l5 depend upon the capacitances of electrodes 4, 5, l, 5' respectively, the potential of each deflection plate is a function ofthe capacitance between one of the electrodes and the conductor 6 o! the cable 3.

The polarities of the re'ctiflers I8, 11, It, n'

and the connections to the deflection plates 20, 21,20, 2| of the oscillograph are so arranged that the fluorescentspot on the screen of the oscillograph moves in the same direction as the conductor 5 when it moves from the central powhen the conductor 6 moves .toward electrode 5, the capacitance of' electrode 5 is increased and the potential of plate 2| is decreased; the capacitance of electrode 4 is decreased and the potential 01. plate 20 is increased. Therefore the cathode-ray is deflected'away from plate 2| and sition within the cable 3. Thus, in the diagram,

toward plate 20, that is, it -moves in the same directionas the conductor 6. Similarly, .the capacitances of electrodes 3' and 5' control the potentials of plates 20' and 2|', respect ively, and the cathode-ray beam moves approximately in the same direction as the conductor 6 'no matter what that direction may be.

With'linear amplification the deflection of the cathode-ray beam is not a linear function of the amount 01- displacement of the conductor 3 1mm its central position, since the capacitance'oi' any one of the electrodes 4, 5, I, 5', with respect.

to the conductor 6 of the cable 3 isnot a linear functicn'oi the distance between them. everfsuch distortion'as is present is not important in the uses for which the apparatus is 2c, 25', :1, 21', 2a, 20'. They may be replaced by suitably designed power supply circuits to operate from available alternating current. Resistances 29, 30, 29', 30 are thdplate resistors of the amplifying tubes l8, l9, l8, l3.

Capacitors 3|, 32, 3|", 32' are the customary capacitances used in rectifying circuits to suppress ripple in the rectified voltage. These capacitors, together with the resistors 33, 33, 33', 34', control to a large extent the time constants of the circuit. The faster the cable 3 is drawn through the electrodes 4, 5, I, 5, the lower must be the time constants.

Registration of the location of thesconductor 6 within the cable 3 may be obtained by taking a moving picture record of the spot on the screen of the oscillograph, or by recording the potentials of points I4, l5, l4, IS, with or without amplification and rectification, with potential recorders.

Means for painting or otherwise marking portions of the cable 3 wherein the conductor 6 is not properly centered, may be controlled by relays actuated directly by the potentials of the" points l4, l5, l4, I! with or without amplification or rectification, or by relays actuated by a photoelectric-cell or cells so arranged that they are actuated when the fluorescentLspot on the screen of the oscillograph leavesdts central position.

- Similarly, in the process of fabricating the -mental purposes without the payment of any Howintended. Non-linear amplification may be used -"to reduce the distortion to any desired minimum.

purpose of rectifying the outputs of the,

bridges is to produce steady potentials on "the deflection plates 20, 2|, 20", 2| of the oscillograph."Rectiflcation could be dispensed with,

A oscillograph.

Resistors 22, 23 and 22323 are used as a convenient means of maintaining points 24- and 24' respectively, at the same potentials as the control 'grids of. the amplifying tubes l8, l9, l8,

- 19' when the bridges are balanced. Capacitors 2 5. 25' are inserted in the circuit to suppress oscillations in the amplifiers l3, I3, |3', |9' and may not always be necessary. Grid bias, screen,

..and plate potentials are obtained from batteries royalties thereon or therefor.

What we claim is:

1. An apparatus for determiningthe position of the conductor in an insulated cable, comprising; two pairs of electrodes, the electrodes of each pair being placed on opposite sides of the cable, said electrodes in the aggregate forming a, hollow segmented cylinder throughwhich the cable may be drawn, a bridge circuit, connecting the electrodes 01' each pair with oppositely located' deflection plates of a cathode-ray oscillograph, the relative positions of said deflection plates being the same as those -of their corresponding electrodes, and means in said bridge circuits to vary thipotentialsof eachadeflection plate in proportion to changes of capacity between its corresponding electrode and the conductor, whereby the cathode-ray beam will move in a corresponding direction and inan amount proportional to lateral movement of the conductor in the cable. 4

2. An apparatus for determining the position tor in parallel with said capacitance, rectifying means for converting the potential originating in each of said si'desof said bridges to direct current potentiahga cathode-ray, oscillograph,

means connecting the opposite sides of each of said bridges to apair of the deflection platesof bridges, the sides of each of said bridges consisting of an electrical resistance and the capaci tance between the said conductor and an electrode in spaced relation to said cable, and a variable capacitor in parallel with said capacitance;

rectifying means for converting the potential to direct current, a cathode-ray'omogl'aph,

10 in said cable.

' means connecting the opposite sides of each qt said bridges to a pair oflthe deflection plates of said oscillograph, and amplifying means for raising said potentials to values suflicient to operate said deflection plates, whereby the position 01 the fluorescent spot on the screen of said oscillograph will indicate the positionol' said conductor B. GODFREY.

originating in each of said sides 9mm bridges 

